We usually use this before tickets are even created. In many cases, an RFC can lead to an epic-level ticket that includes multiple user stories.

In other cases, it can be very tactical. I think you are right, probably it can be expressed directly in the form of a ticket. The discussion well happen in the comments to the ticket in this case.

A benefit of the RFC approach is if it lives in the repo itself the documentation is along side the code.

Anything that will prompt stakeholders to engage and clarify expectations is a win. It's hard to make this happen even when everybody is philosophically aligned on the need to do so, so reframing it as an RFC could be quite the useful "One Weird Trick" ;-).